1. Field of the Invention
The invention relates generally to integrated circuit semiconductor devices and more specifically to amplifiers for very high radio frequency operation.
2. Description of the Prior Art
Emitter and source-coupled pairs are perhaps the most widely used two-transistor subcircuits used in monolithic analog circuits. The usefulness of this circuit stems from the facts that cascades of emitter-coupled pairs of bipolar transistors or source-coupled pairs of FETs can be directly coupled to one another without interstage coupling capacitors and that the differential input characteristics provided by the emitter-coupled pair are required in many types of analog circuits. The simplest form of emitter-coupled pair includes two transistors with their emitters connected together and their respective collectors individually connected to a positive supply potential through respective resistors. The biasing circuit in the common emitter lead can be either a simple resistor, or a transistor current source.
Monolithic integrated circuits (ICs) that include radio frequency (RF) amplifiers operating above one gigahertz and with very low signal levels can benefit from an all-differential signal processing configuration. For example, RF and intermediate frequency (IF) amplifiers with differential inputs and outputs exhibit better noise immunity and tend to generate less spurious noise. Emitter-coupled transistor pairs naturally accept a differential input between the respective transistor bases, and produce a differential output between the transistor collectors. However, conventional transistor current sources used in biasing emitter-coupled pairs behave in less than perfect ways at high frequencies, e.g., above one gigahertz.
Differential amplifiers with applications that limit their operation to only very high frequencies above one gigahertz can employ coupling capacitors between amplifier stages that are simple and practical to implement directly on an IC using standard fabrication techniques. Typically, capacitors ranging from twenty-five femtofarads to one picofarad are necessary. Therefore, the inherent benefit of emitter-coupled transistor pairs, e.g., direct coupling and elimination of coupling capacitors, is relatively unimportant.
Poor RF performance in amplifiers with single-input to differential-output, and differential-input to differential-output, configurations due to inadequate transistor current source biasing, can be corrected by a base-input transistor amplifier and an emitter-input transistor amplifier combined in a pair with appropriate integrated capacitor coupling, such as is done in the present invention.